NATIVE PLANTS AS WEEDS
Most farmers are quite comfortable with calling the native plants trying to re-establish in their pastures from nearby bush weeds (in the sense of a plant growing where it is not wanted), but to a gardener, the idea of a "native weed" may seem a contradiction in terms. However, there is really no difference in the impact made by a plant which gets out of a garden and into the bush whether it comes from overseas or has its origin somewhere in Australia. Either can displace locally native species and alter the habitat for native animals, sometimes drastically.

Australian plants have been widely promoted as garden subjects only in the last twenty or thirty years, and a great many species have been brought into cultivation only recently. Many plants are grown outside the part of the country in which they occur naturally. Some of the species which have been in cultivation the longest are now proving themselves to be quite as invasive as some of the worst plants from overseas, and many more Australian plants are likely to show invasive behaviour in time. The crucial question in choosing plants for a garden should be not "where does it come from" but "how likely is it to spread". Gardeners often choose natives on the grounds that they are hardier, and require less watering. This may be true of some species, but is not invariably true. There are many non-invasive exotic plants with low water requirements.

Following are some examples of Australian natives which commonly spread into bush around towns:

Bluebell creeper (Sollya heterophylla) is a climber or twining shrub from Western Australia that has been planted in the eastern states for a long time for its attractive blue flowers. However, it has bird-distributed fruits, which make it easy for this species to spread well beyond the gardens in which it is planted. It has become very widespread in sandy coastal areas around Melbourne, a habitat which probably closely resembles its natural habitat in Western Australia. It has been observed naturalising in several locations on the NSW south coast, and is likely to become more widespread in time. Around Melbourne it can form dense thickets which clamber over other shrubs excluding light and eventually smothering them, in much the same fashion as do exotic vines such as honeysuckle and bridal creeper. Because it has evolved on the poor soils of Australia it is likely to invade a much wider range of habitats than the exotic vines, many of which are restricted to the better soils found along creek banks and in other moist sites.

Wattles (Acacia species) commonly spread beyond their original planting site, although their seed is only occasionally spread by birds. It more usually travels only short distances with the assistance of strong winds, ants and explosive release of the seeds from the pods on hot days. However, because wattles are widely planted in farm windbreaks and on roadsides beyond towns, they still have considerable potential to spread into local native vegetation. Cars and machinery can transport the seed in mud on tyres and undercarriage. A key characteristic of wattles is their hard-coated seed which remains viable in the soil for many years, if not decades, and can require some form of disturbance such as fire or earthworks before it will germinate. Wattles can therefore represent something of a time-bomb in the natural environment, in that they may not declare themselves as being invasive until after such a disturbance event, when suddenly their seedlings are everywhere. Dense stands of wattles can out-compete other native vegetation for soil moisture and nutrients, causing loss of groundcover biodiversity and suppressing regeneration of other trees and shrubs. The species of wattles which become weedy will vary from place to place, depending on which species are most frequently cultivated in that area. The two most frequently naturalised species on the south coast currently are Cootamundra wattle (Acacia baileyana) and Queensland silver wattle (Acacia podalyriifolia), for the simple reason that these species have been in cultivation for longer than any others, because of their attractive silver foliage colour and winter-blooming habit. However, many more species have been found to naturalise in other areas, particularly around cities, where the number of gardeners, and hence the number of species found in cultivation, is much higher. All non-local wattles have the potential to become weedy.

Even some local species can become weedy. The most obvious case is coast wattle (Acacia sophorae). This plant formerly only grew on sand or occasionally sea cliffs very close to the sea. If planted outside this narrow habitat range, it can spread into other types of vegetation, where it may come to dominate the understorey, to the detriment of the other plants living there. It can also spread naturally and in Eurobodalla it often spreads onto previously grassy headlands. There are several of these headlands on basalt derived heavy soils which appear to have been treeless prior to European arrival. They were grazed or even cultivated during early settlement of the region, which kept them open and grassy. Now that grazing has been withdrawn because of the breaking up of farms for residential development, most are being overrun by coast wattle and other salt-tolerant shrubs and small trees. This is likely to eventually cause the loss of these unique grassy habitats, and the local extinction of a rare plant, Austral toadflax (Thesium australe), that lives in them. Before European arrival they were probably kept open by a combination of high exposure to salt-laden winds and regular burning by Aboriginal people.

Sweet pittosporum (Pittosporum undulatum) is a local native species which has been popular as a garden plant for a long time because of its lush European appearance and sweet-scented flowers. Its natural range is the coastal strip from the south-east corner of Queensland to the eastern side of Melbourne. Its use in gardens has enabled it to spread into western Victoria and South Australia as well. The seed is spread by birds. Within its natural range it is becoming much more common in forest than it used to be. This is largely due to less frequent burning in the last twenty years or so, but has probably also been helped by its use in gardens. Pittosporum is fire-sensitive, and in areas which are burnt every few years it is restricted to the less fire-prone sites such as moist gullies, where it may become a medium-sized rainforest tree. Without burning it spreads out into the rest of the landscape. It is tolerant of a very wide range of soil types and soil moisture.

The negative impacts of pittosporum come from the fact that it castes a dense shade which suppresses all other growth underneath it. It is relatively non-flammable and by suppressing all groundcover, it makes infested areas very difficult to burn. Dense infestations of pittosporum can therefore turn into a monoculture of this species, with emergent eucalypts. Eventually the eucalypts may even die out because of changes to the soil conditions or other factors, and are unlikely to be replaced because their seedlings are intolerant of shade.

The dense understorey of pittosporum is very attractive to the aggressive native bird, the bell miner (or bellbird). This species is naturally occurring locally but seems to have increased in numbers in recent years. It is very common on the far south coast, but less common in the northern part of the region. Colonies of bell miners drive out most other birds from their territory, and by doing this they decrease the overall consumption of insects in the area. The eucalypts within their territory become over-burdened with sap-sucking and leaf-chewing insects, and as a result their crowns begin to look very thin, and in time they may even die. The understorey, freed from shading and competition for moisture from the trees, becomes very thick. It is not known to what extent this is a natural process, and to what extent it has been influenced by the activities of humans. Bell miners seem to like forest edges and activities such as partial clearing for residential subdivisions create many forest edges. The planting of numerous flowering shrubs such as grevilleas in such areas may also advantage the bell miners, which belong to the honeyeater group of birds, although their main diet is insects.

Pittosporum is regarded as an environmental weed and is removed by bush regenerators in Sydney and Melbourne in the same way as exotics with similar behaviour such as privet. In areas where it is not naturally common it would be preferable not to grow pittosporum in gardens, but in areas where it is common in the bush, a few garden plants are not likely to make a significant difference. It should not be used in public plantings or in revegetation sites. Consideration needs to be given to managing pittosporum by increased burning in some coastal vegetation types.

However, it should also be mentioned that pittosporum is a natural component of the far south coast plant communities Brogo Wet Vine Forest and Dry Rainforest which are listed as Endangered Ecological Communities under the Threatened Species Conservation Act. These communities occur in areas which have a naturally low fire frequency because of an abundance of large rock outcrops. These inhibit the spread of fires through such sites. It is also a natural component of Littoral Rainforest, which enjoys the protection of State Environmental Planning Policy (SEPP) 26, and of most other rainforest types found on the south coast. It can be the first colonising species which begins the transition from eucalypt forest to rainforest, in some fire-free situations. However, in most situations where pittosporum invades it is unlikely that the site will become a viable rainforest stand, because there is not sufficient long-term protection from wildfire. Sites on which rainforest develops and persists must have more or less permanent fire protection, since many rainforest plants are killed by fire.

Coast teatree (Leptospermum laevigatum) is another plant whose natural distribution includes the south coast, although it is not naturally occurring in much of that area. It is confined to situations very close to the coast on dunes and headlands, and even in these situations there are many parts of the coast where it does not occur. It is sold by nurseries for gardens and rural windbreaks, where it probably does no harm, as it is not very likely to reproduce in these situations, and its fine seed does not spread very far. However, it is also planted by Dunecare and similar groups to stabilise sand in locations where it would not have naturally occurred in the past, and it may occasionally find its way into plantings made to stabilise riverbanks. Coast teatree is another plant which can substantially change the environment for other local native plants, by forming dense thickets and eliminating other plants by shading them or depriving them of water. It can invade coastal heaths or coastal forests, resulting in the loss of many plant species and their replacement by a dense monoculture of coast teatree. This can also have severe impacts on the animals using these coastal habitats. Coast teatree has become a serious environmental weed in Western Australia and South Africa, as well as within its natural distribution in Victoria.